1. Field of the Invention
The present invention relates to a pressure signal generator for detecting a pressure or the like of various types of process fluids. More specifically, the present invention is directed to a pressure signal generator wherein a variation in pressure at an overload pressure protective operation point caused by an ambient temperature change is compensated to improve measurement performance.
2. Description of the Prior Art
Conventionally, e.g., when a flow rate of a fluid in a tube is to be measured, an orifice plate is provided in the tube as a fluid resistance, and a pressure difference beween the upstream and downstream side fluids with respect to the plate is calculated based on a predetermined calculation formula. In a differential pressure signal generator, for measuring the pressure difference, right and left measuring chambers are defined by a center diaphragm in a detector body. High and low pressure-side measured pressures are supplied to the right and left measuring chambers through respective barrier diaphragms. A movement of a fill liquid sealed in the measuring chambers caused by the pressure difference is converted to an electrical output by a strain in a semiconductor sensor or the like which is provided in a fill fluid circuit.
In the differential pressure signal generator having the above arrangement, when an excessive or overload pressure exceeding a predetermined pressure is applied to a high or low pressure-side barrier diaphragm, a fill liquid sealed between the corresponding diaphragm and the detector body-side pressure-receiving surface as a pressure transmitting medium flows to the center diaphragm side, and the barrier diaphragm is caused to contact a body pressure-receiving surface, so that a pressure exceeding the predetermined pressure will not be applied to a pressure sensor. The pressure at an excessive pressure protective operaiton point is set to be smaller than a maximum pressure capability of the pressure sensor in the signal generator. Concurrently, a maximum measurable pressure of the signal generator is determined to be lower than the pressure at the excessive pressure protective operation point.
In the differential pressure signal generator having the above arrangement, however, the fill liquid, such as silicone oil, sealed in the detector pressure measuring chambers of the detector body as the pressure transmitting medium is expanded or contracted by an ambient temperature change, and its volume is changed by the expansion/contraction. This changes the internal pressure of the detector pressure measuring chamber, thus causing a pressure at an excessive pressure protective operation point to fluctuate greatly. When the variation in pressure at the excessive pressure protective operation point is large, the maximum measurable pressure of the signal generator must be set to be lower than the maximum pressure capability of the sensor by the variation thereby reducing the measuring performance of the signal generator. The same problem occurs in a pressure signal generator which detects a pressure of various types of fluids by using a detector pressure measuring chamber containing a fill liquid.